In-Plane Effective Diffusivity in PEMFC Gas Diffusion Layers

Predicting the Topological and Transport Properties in Porous Transport Layers for Water Electrolyzers

The porous transport layer (PTL) in polymer electrolyte membrane (PEM) electrolyzers governs the overall efficiency. Its structural, thermal, and electronic properties determine how effortlessly the gases can be produced and can exit the PEM electrolyzer. In this study, we apply a stochastic reconstruction method for titanium felt-based PTLs to generate PTLs with different porosity, fiber radii, and anisotropy parameters. The morphology and topology of these PTLs are numerically characterized, and transport properties, such as gas diffusion coefficients and electrical and thermal conductivity, are computed via pore-scale modeling. Customized graded PTLs are proposed, exhibiting the optimal topology and bulk structure for the removal of gases, the conductance of electrons, and the transport of heat. The results indicate that the surface and transport properties of PTLs can be tailored by certain morphology parameters: PTLs with lower porosity and smaller fiber radii feature a more sufficient interfacial contact and superior electrical and thermal conductivity. Lowering the anisotropy parameters of PTLs results in a slight loss of interfacial contact but a substantial increase in the electrical and thermal conductivity in the through-plane direction. We outline that the design of PTLs should be differentiated depending on the operating conditions of electrolyzers. For nonstarvation conditions, PTLs should feature low porosity and small fiber radii, whereas for starvation conditions, PTLs should feature high porosity, low anisotropy parameters, and small fiber radii. Furthermore, graded PTLs with enhanced structural and transport properties can be developed by customizing the porosity, fiber radius, and fiber orientation.

Effect of Thickness and Outlet Area Fraction of Macroporous Gas Diffusion Layers on Oxygen Transport Resistance in Water Injection Simulations

Enhanced water removal through the gas diffusion layer (GDL) is important for the design of high-performance proton exchange fuel cells. In this work, the effects of GDL thickness and open area fraction at the GDL/flow field interface are examined under water invasion for a carbon-paper GDL (similar to Toray TGP-H series). Both uncompressed and inhomogeneously compressed samples are considered. Transport in heterogeneous, macroporous GDLs is modeled by means of a hybrid 3D discrete/continuum formulation based on a subdivision of the porous medium into control volumes due to the lack of a well-defined separation between pore and layer scales. Capillary-dominated transport of liquid water is simulated with an invasion percolation algorithm, while oxygen diffusion is simulated with a continuum formulation. Model predictions are validated with previous numerical and experimental data. It is shown that the combination of thin GDLs ($$\mathrm {thickness} \sim 100\; \mu \mathrm {m}$$ thickness ∼ 100 μ m ) and high GDL/flow field open area fractions can facilitate water removal/oxygen supply from/to the catalyst layer and can provide a more uniform oxygen distribution over large cell active areas. In agreement with previous work, porous flow fields with pore sizes comparable to the GDL thickness are good candidates to meet the above requirements, while improving water removal from the flow field (higher gas-phase velocity than conventional millimeter-sized channels) and ensuring a more uniform assembly compression.

Stochastic Analysis of the Gas Flow at the Gas Diffusion Layer/Channel Interface of a High-Temperature Polymer Electrolyte Fuel Cell

Gas diffusion layers (GDLs) play a significant role in the efficient operation of high-temperature polymer electrolyte fuel cells. They connect the electrodes to the gas channels of the bipolar plate by porous material with a meso-scale geometric structure. The electrodes must be sufficiently supplied by gases from the channels to operate fuel cells efficiently. Furthermore, reaction products must be transported in the other direction. The gas transport is simulated in the through-plane direction of the GDL, and its microstructure created by a stochastic model is equivalent to the structure of real GDL material. Continuum approaches in cell-scale simulations have model parameters for porous regions that can be taken from effective properties calculated from the meso-scale simulation results, as one feature of multi-scale simulations. Another significant issue in multi-scale simulations is the interface between two regions. The focus is on the gas flow at the interface between GDL and the gas channel, which is analyzed using statistical methods. Quantitative relationships between functionality and microstructure can be detected. With this approach, virtual GDL materials can possibly be designed with improved transport properties. The evaluation of the surface flow with stochastic methods offers substantiated benefits that are suitable for connecting the meso-scale to larger spatial scales.

Capillary Hysteresis in Neutrally Wettable Fibrous Media: A Pore Network Study of a Fuel Cell Electrode

Hysteresis in the saturation versus capillary pressure curves of neutrally wettable fibrous media was simulated with a random pore network model using a Voronoi diagram approach. The network was calibrated to fit experimental air-water capillary pressure data collected for carbon fibre paper commonly used as a gas diffusion layer in fuel cells. These materials exhibit unusually strong capillary hysteresis, to the extent that water injection and withdrawal occur at positive and negative capillary pressures, respectively. Without the need to invoke contact angle hysteresis, this capillary behaviour is re-produced when using a pore-scale model based on the curvature of a meniscus passing through the centre of a toroid. The classic Washburn relation was shown to produce erroneous results, and its use is not recommended when modelling fibrous media. The important effect of saturation distribution on the effective diffusivity of the medium was also investigated for both water injection and withdrawal cases. The findings have bearing on the understanding of both capillarity in fibrous media and fuel cell design.